As a conventional fixing apparatus installed in image forming apparatuses of the electrophotographic system, a fixing apparatus of a roller pair system has been available that includes a fixing roller and a pressing roller.
The fixing roller includes, for example, a hollow metal core of aluminum or the like coated with an elastic layer, and a halogen lamp is installed as a heat source inside the metal core. In this type of fixing roller, the halogen lamp is turned ON/OFF with a temperature control circuit in response to a signal from a temperature sensor provided on the surface of the fixing roller. In this way, the surface temperature of the fixing roller is maintained constant.
The pressing roller includes a metal core with a heat-resistant elastic layer, such as a silicon rubber, which is coated around the metal core. When the pressing roller and the fixing roller are pressed against each other, a predetermined nip region is formed as the elastic layer undergoes elastic deformation. In the fixing apparatus of the roller pair system, a sheet of paper with a transferred unfixed toner image is passed through the nip region and the toner image is thermally fixed on the sheet by fusing the toner with heat.
There is a drawback in common fixing apparatuses of the roller pair system. When continuously feeding more than one sheet at high speed, the surface temperature of the fixing roller drops greatly, and this makes it difficult to maintain the surface temperature of the fixing roller constant. This problem is attributed to the fact that the generated heat inside the metal core is transferred to the toner image via the elastic layer (for example, silicon rubber) with poor heat conductivity, and because of this it takes a considerable amount of time for the heat of the heat source to be transferred to the surface of the fixing roller.
As a countermeasure, there has been proposed a fixing apparatus of an external heating system, which includes an external heating section that heats the fixing roller from outside by being brought into contact with the periphery of the fixing roller.
For example, Patent Publication 1 (Japanese Laid-Open Patent Publication No. 64-52184 (published on Feb. 28, 1989) discloses a fixing apparatus that includes: first detecting means for detecting a surface temperature of a heat roller in which a heater is installed; second detecting means for detecting a temperature of an external heating section that heats the heat roller; and a control section for controlling the external heating section based on results of detection by these detecting means. The control section includes a bridge circuit connected in parallel to the first detecting means and the second detecting means; a comparator for comparing the output of the bridge circuit with a reference voltage; and ON/OFF means for turning ON/OFF the external heating section based on the output of the comparator.
Further, Patent Publication 1 describes detecting a temperature drop on the surface of the heat roller as caused by feeding, and compensating for it with the external heating section. As described in this publication, controlling the external heating section according to the result of temperature detection alleviates fluctuations of fixing temperature due to changes in amount of feeding.
However, in the technique disclosed in Patent Publication 1, the first detecting means and second detecting means are connected to each other in parallel, and a mean resistance of these two detecting means is used for feedback in the temperature control of the external heating section. As such, the temperature control does not reflect individual results of temperature detection for the fixing member and the external heating section, with the result that the temperatures of these members cannot accurately be controlled at target temperatures.
The temperature control of the external heating section may be made based solely on the result of detection of surface temperature of the external heating section. However, in this case, the surface temperature of the fixing member may gradually increase and exceed the pre-set paper feed temperature when, for example, intermittently feeding a single sheet of paper (making single copies), owning to the fact that the temperature of the fixing member is not monitored. Alternatively, the temperature control of the external heating section may be made based solely on surface temperature of the fixing member. While this may be able to maintain the surface temperature of the fixing member within a predetermined temperature range when the number of feeds is small, it causes delayed detection and generates undershoot in the early stage of feeding during a continuous paper feed.
In a color image forming apparatus, the external heating section requires accurate temperature control when the monochromatic mode and the color mode have different processing speeds (feeding speeds). The following describes this in detail.
When the monochromatic mode and the color mode have different processing speeds (feeding speeds), the fixing temperature (temperature of the fixing member) also differs between these modes. As such, in these modes, there is only a narrow temperature range (common non-offset range) in which fixing defects or offset (low-temperature offset or high-temperature offset) due to excessively low fixing temperatures do not occur (in some cases, such common non-offset range may not exist at all). For example, the color mode requires a fixable temperature range of 160° C. to 190° C., inclusive (fixing defects occur below 160° C., and high-temperature offset occurs above 190° C. If the monochromatic mode requires a fixable temperature range of 180° C. to 210° C., inclusive (fixing defects occur below 180° C., and high-temperature offset occurs above 210° C.), the common non-offset range is confined between 180° C. and 190° C., inclusive.
In this case, the fixing member needs to be maintained throughout the standby period at a temperature that allows for feeding in the both modes, in order to allow a quick start for the feeding operation. More specifically, the temperature needs to be maintained between the lower limit of the fixable temperature for the monochromatic mode (180° C. in the foregoing example) and the upper limit of the fixable temperature for the color mode (190° C.). Further, in order to allow the image formation modes to be switched quickly, the temperature of the fixing member also needs to be confined in this temperature range.
The surface temperature of the fixing member can be controlled more accurately by varying the surface temperature of the fixing member through the temperature control of the external heating section brought into contact with the surface of the fixing member, rather than varying the surface temperature through the temperature control of the heat source provided inside the fixing member. Thus, temperature control of the external heating section is particularly important when the temperature of the fixing member needs to be accurately controlled within a narrow temperature range as in the foregoing case where the monochromatic mode and the color mode use different processing speeds (feeding speeds).